Volume 90, Issue 8, 15 April 1989
Index of content:
Sub‐Doppler direct infrared laser absorption spectroscopy in fast ion beams: The fluorine hyperfine structure of HF+90(1989); http://dx.doi.org/10.1063/1.455800View Description Hide Description
We report the development of a new general technique for measuring vibration–rotation spectra of molecular ions with sub‐Doppler resolution and with accurate determination of the mass and number density of the carriers of all spectral features. With this method, called direct laser absorption spectroscopy in fast ion beams (DLASFIB), we have carried out the first observation of direct absorption of photons by ions in a fast ion beam. Hyperfine‐resolved vibration–rotation transitions of HF+ have been measured, and along with optical combination differences and laser magnetic resonance data, have been analyzed to yield the fluorine hyperfine parameters a, b, c and d for both v=0 and v=1 in the X 2Π state. Comparisons with many‐body perturbation theory results are presented.
The electronic spectrum of NOCl: Photofragment spectroscopy, vector correlations, and a b i n i t i o calculations90(1989); http://dx.doi.org/10.1063/1.455801View Description Hide Description
The electronic absorptionspectrum of NOCl in the region 620–180 nm is assigned by using vector properties of the NO photofragment and the results of a b i n i t i o calculations at the CI level. In assigning the electronic spectrum, we take into account the recoil anisotropy, rotational alignment, and Λ‐doublet populations of NO, as well as the calculated vertical excitation energies, oscillator strengths, and the nature of the orbitals involved in the transitions. In the experiments, we use expansion‐cooled samples and measure the recoil anisotropy parameters from the Doppler profiles of selected NO A 2Σ+←X 2Π rotational lines. The alignment parameters and Λ‐doublet populations are derived from the rotational spectra using different laser polarizations and excitation–detection geometries. The theoretical calculations treat all low‐lying singlet and triplet states. The calculations yield least energy paths for the excited states, with optimized r NO and ClNO angle as a function of r ClN, as well as the angular dependences of the potentials and oscillator strengths of the singlet–singlet transitions. The following assignments are proposed for the main absorption bands: (1) E band—T 1(1 3 A‘)←S 0(1 1 A’); the transition borrows intensity by mixing with remote singlet states, predominantly the 4 1 A’ state; (2) D and C bands—S 1(1 1 A‘)←S 0(1 1 A’); the C band corresponds to excitation of ν’ 1 in S 1; (3) B band—S 3(2 1 A’)←S 0(1 1 A’); (4) A band—S 5(4 1 A’)→S 0(1 1 A’). The assignments proposed here are in full agreement with all the experimental observations and the results of the calculations. Despite the shallow minima calculated for the T 1 and S 1surfaces,dissociation on all the surfaces is fast, and the implications of the results to the dissociationdynamics are discussed.
Selective excitation and analysis of NaF:Cu+ electronic transitions using two‐color, two‐photon spectroscopy90(1989); http://dx.doi.org/10.1063/1.455802View Description Hide Description
Nonresonant, one‐color and two‐color, two‐photon excitation spectra of d 1 0→d 9 s transitions in NaF:Cu+ at T<15 K are presented. The two‐color method is shown to provide a means of using the symmetry selectivity of two‐photon excitation to its fullest extent to separate cleanly bands normally distorted or obscured by other transitions. The spectra give unambiguous assignments and accurate energies of the T 2g (3 T 2g ) and E g (3 T 2g ) electronic states within the d 9 sexcited statemanifold. Vibronic fine structure is observed at the electronic origins of transitions to each of these states. This structure aids in band assignment and in analysis of the symmetric mode vibrational potentials in the excited states. Band intensities are analyzed and vibronic fine‐structure linewidths are interpreted in terms of nonradiative relaxation. These new data require a reassessment of the assignment of the bands previously attributed entirely to transitions to the 1 E g state. It is suggested that Jahn–Teller splitting does not distort this state to a spectroscopically observable degree, contrary to earlier interpretations of the spectra, and that only the lower energy of the two E g bands originates from transitions to the state ofprimarily singlet character. Observed spin–orbit coupled energy levels are modeled using crystal field splittings, singlet–triplet splittings, and the Cu+3d spin–orbit constant as variable parameters. The resulting fit gives energies and intensities which are in agreement with all measured experimental values if ζ3d is approximately equal to its free ion value.
90(1989); http://dx.doi.org/10.1063/1.455803View Description Hide Description
We report multiplet‐specific photoionization cross sections and photoelectron angular distributions for the 3σ g orbital of O2 leading to the b 4Σ− g and B 2Σ− g ion states obtained using Hartree–Fock photoelectron orbitals. These cross sections show significant nonstatistical behavior at low photoelectron energies which arises from the sensitivity of the kσ u shape resonance to the exchange potentials of these two molecular ions. Specifically, the oscillator strength associated with the shape resonance in the B 2Σ− g cross section is shifted to lower energy compared to that of the b 4Σ− g cross section. This shift gives rise to a quartet to doublet cross section ratio of more than 15:1 near threshold. These effects are difficult to assess in the measured cross sections due to the presence of strong autoionization features. Significant multiplet dependence is also seen in the calculated photoelectron angular distributions.
Measurement of the perpendicular rotation‐tunneling spectrum of the water dimer by tunable far infrared laser spectroscopy in a planar supersonic jet90(1989); http://dx.doi.org/10.1063/1.455804View Description Hide Description
Fifty‐six transitions from the K=1 lower→K=2 lower tunneling–rotation band of water dimer have been measured and assigned at 22 cm− 1 by direct absorption spectroscopy in a cw planar supersonic jet expansion using a tunable far infrared laser spectrometer. Two different models were used to fit the data and several spectroscopic constants were determined for the upper and lower states. This work supports the local IAM model recently proposed by Coudert and Hougen for the hydrogen bond tunneling dynamics of the water dimer. This model includes four different tunneling motions, all of which contribute to the observed tunneling splittings. This is the most complicated hydrogen bonded system considered to be well understood at this time, at least in the lowest few K states.
90(1989); http://dx.doi.org/10.1063/1.455805View Description Hide Description
Gas‐phase fundamental and CH and CO overtone spectra (700–17 500 cm− 1) of 2,2,2‐trideuteroacetaldehyde were recorded using FTIR and laser photoacoustic techniques. The Fermi resonance structure in the overtone spectra of the coupled CH stretching and in‐plane CH bending vibrations is analyzed with a tridiagonal Hamiltonian, yielding a large effective coupling constant, ‖k ’ s b b ‖ =93 cm− 1, corresponding to subpicosecond redistribution times. No coupling between the out‐of‐plane CH bending mode and the Fermi resonance system is apparent. This study presents the first detailed analysis of the anharmonic couplings in the CH chromophore at an sp2carbon atom. The in‐plane CH bending vibration couples in a manner similar to the CH(sp3) bending vibrations, whereas the out‐of‐plane bending vibration is decoupled, similar to the CH(sp) bending vibrations.
State mixing and vibrational predissociation in large molecule van der Waals complexes: t r a n s‐stilbene–X complexes where X=He, H2, Ne, and Ar90(1989); http://dx.doi.org/10.1063/1.455806View Description Hide Description
We report a detailed study of vibrational predissociation and intramolecular–intermolecular state mixing in the first excited singlet state of t r a n s‐stilbene van der Waals complexes with helium, hydrogen, neon, and argon. We present evidence that the helium atom in stilbene–He and the H2 molecule in stilbene–H2 possess very low frequency van der Waals bending levels involving delocalization of the complexed species over both phenyl rings. In stilbene–He, the mode‐selective, strong coupling of the out‐of‐plane phenyl ring modes with the pseudotranslation van der Waals modes leads to a dramatic, inhomogeneous broadening of the transitions to several times their breadth in in‐plane vibrations. The observed dispersed fluorescence spectra give product state distributions and internal clock lifetime estimates which can only be made consistent with direct lifetime measurements by assuming extensive state mixing of the intramolecular levels with the van der Waals levels in which the states accessed by the laser are actually only about 30% intramolecular in character. We conclude that in these complexes the processes of intramolecular–intermolecular state mixing (static IVR) and vibrational predissociation are not independent processes but are
closely tied to one another. In fact, the vibrational product state distributions observed for the out‐of‐plane phenyl ring levels can best be interpreted as reflecting the percentage van der Waals character in the initially prepared state. In stilbene–H2 the mode selective coupling exhibits itself as a splitting of the out‐of‐plane transitions into a set of 5–6 closely spaced transitions separated by only about 1 cm− 1. The sequence of transitions is suggestive of an in‐plane potential for H2 motion which is nearly flat across the entire length of the stilbene molecule with a small barrier presented by the ethylenic carbons through which the H2 molecule can tunnel. Dispersed fluorescence spectra from these levels point to a two‐tiered coupling scheme with the bound van der Waals levels. In contrast, the out‐of‐plane phenyl transitions in stilbene–Ne and stilbene–Ar possess unusual shifts, but the transitions are narrow once again. In these cases the complexed atom appears to be largely localized over a single phenyl ring.
State mixing and internal rotation predissociation in large‐molecule van der Waals complexes: p‐methyl‐t r a n s‐stilbene–X where X=He, H290(1989); http://dx.doi.org/10.1063/1.455807View Description Hide Description
In this paper we extend our study of state mixing and predissociation in large‐molecule van der Waals complexes by considering the p‐methyl‐t r a n s‐stilbene–X complexes where X He, H2. The addition of the methyl group in the para position makes the two phenyl rings inequivalent but does not significantly change the ν37 vibrational frequency. It also introduces internal rotation levels as an added source of low frequency transitions which can be coupled to the van der Waals modes. The p‐methyl‐t r a n s‐stilbene molecule possesses two clear progressions on which we focus much of our attention: an internal rotation progression 2e→3a 1 →4e, and a progression in ν37 activity: 3a 1,371 03a 2, 372 0. The p‐methyl–He transitions built on the internal rotor levels exhibit narrow rotational band contours. Internal rotation predissociation from these levels appears to be efficient. The transitions in the ν37 progression in p‐methyl–He show both increasing breadth and decreasing shift as one increases the out‐of‐plane vibrational excitation. The band contour of the 372 0 0a 1 transition is very
similar in breadth and shape to the 372 0 transition in stilbene–He. This broadening can only be accounted for by assuming, as we did in t r a n s‐stilbene–He, that the He atom can move almost freely between the two phenyl rings. The corresponding transitions in p‐methyl–H2 are no longer split (as they were in stilbene–H2 ), but the dispersed fluorescence spectra from levels below the dissociation limit show less resonance fluorescence and more broadening. We argue that the states carrying the absorption strength from the ground state involve a H2 molecule localized on a single phenyl ring while background van der Waals levels built on lower lying vibrational levels are delocalized. The enhanced state mixing in p‐methyl–X is a consequence of two factors. First, the reduced symmetry of the p‐methyl–X complex makes coupling with all van der Waals states allowed. Second, the internal rotor levels act as added base levels on which stacks of van der Waals levels can be built, thus increasing the overall density of van der Waals states at a given energy.
90(1989); http://dx.doi.org/10.1063/1.455808View Description Hide Description
Magnetic deflection spectra of beams of pure NO2 and NO2 seeded into rare gases have been determined using a Stern–Gerlach apparatus. Using He as carrier, a simple two‐line deflection pattern is observed, indicating the breakdown of spin–rotation coupling in fields of 10 kG or more. Use of the other rare gases as carriers yields the two satellites at positions dictated by the beam velocities, and in addition, a more‐or‐less intense component at zero deflection which we hold to be due to diamagnetic or weakly paramagnetic (NO2) x clusters. The deflection amplitudes and line shapes are in good agreement with calculations. Additional weak bands observed in the spectra are likely due to van der Waals clusters such as NO2 ⋅ Ar and NO2 ⋅ Ne. 1 4N/1 5N isotope effects in NO2 were visible as changes of line shape.Zeeman level anticrossings are calculated quantum mechanically to occur for NO2 in the 20–30 kG regime, however their effects are small and were not observed. Deflection spectra of NO in its 2Π3 / 2 thermally excited state were obtained showing satellites at the positions predicted assuming pure Hund’s case‐a coupling. Modeling of the observed spectra shows that both the electronic and rotational temperatures of the NO in the beam are rather high (20–125 K). The observed magnetic deflection spectra of the dialkyl nitroxide TEMPO when combined with line‐profile calculations do not support the suggestion of Amirav and Navon [Chem. Phys. 8 2, 253 (1983)] that this molecule undergoes significant intramolecular spin relaxation while in the magnetic gap. In contrast, the SG spectra of the related radical di‐t‐butyl nitroxide shows no magnetic deflection, suggesting a spin lifetime of 0.5 μs or less.
Conformational changes upon S 1←S 0 excitation in 4‐dimethylaminobenzonitrile and some of its chemical analogs90(1989); http://dx.doi.org/10.1063/1.455809View Description Hide Description
One‐color time‐of‐flight mass spectra (mass resolved excitation spectra) for jet‐cooled 4‐dimethylaminobenzonitrile (4‐DMABN) and some of its chemical analogs, dimethylaniline (DMA), 3‐dimethylaminobenzonitrile (3‐DMABN), N, N‐dimethyl‐4‐(trifluoromethyl)aniline (4‐CF3‐DMA), and 4‐(d 6‐dimethylamino)benzonitrile (4‐d 6‐DMABN), are presented and analyzed. Near the origin of the S 1←S 0 transition the low frequency modes can be assigned to motions of the dimethylamino group for this series of molecules. The inversion motion of the dimethylamino group and the dimethylamino group torsion about the Cipso–N bond (the twist coordinate) in S 1 give rise to the most prominent peaks in this spectrum. The potential parameters for the twist coordinate of 4‐DMABN and DMA are quite similar in S 1: B=0.546, V 2=175, and V 4=525 cm− 1 for 4‐DMABN and B=0.546, V 2=175, and V 4=515 cm− 1 for DMA. The V 2 and V 4 terms are slightly larger for 3‐DMABN and 4‐CF3‐DMA. The inversion motion is also similar for these molecules but is more anharmonic for the para‐substituted dimethylanilines, 4‐DMABN and 4‐CF3‐DMA, than for the meta and unsubstituted molecules. A Franck–Condon intensity analysis for the dimethylamino twist in these molecules suggests that this group in 4‐DMABN is displaced in the excited state by ∼30° with respect to its planar orientation in the ground state. In both solutions and monosolvate clusters of 4‐DMABN with polar aprotic solvents, a low lying charge transfer (CT) state is identified in addition to the usual ππ* excited state of the bare molecule. The relation between the bare molecule 4‐DMABN twisting displacement upon excitation and the low lying CT state is discussed.
90(1989); http://dx.doi.org/10.1063/1.455810View Description Hide Description
The photoabsorption and fluorescence cross sections of C2H2 were measured in the 50–106 nm region using synchrotron radiation as a light source. Fluorescence observed at several excitation wavelengths was dispersed to identify the fluorescing species that are excited C2H*, C* 2 , CH*, H*, and possibly C2H+* 2 . The photodissociation process of C2 H2 leading to the formation of fluorescing species is discussed. The C2 (C–A) emission observed at 92.3 and 95.5 nm is produced by the molecular elimination process associated with superexcited state(s). Fluorescence spectra from the two‐photon excitation of C2 H2 at 157.5 and 193 nm were also observed and compared with those of single‐photon excitation at the equivalent excitation energies.
90(1989); http://dx.doi.org/10.1063/1.455811View Description Hide Description
Pulsed laser optogalvanic (LOG) spectra of iodine vapor in a ∼32 MHz rf discharge were excited at 14 900–17 100 cm− 1. Two distinct, time‐resolved components were observed: a fast component, synchronous with the laser pulse, width ∼1 μs, followed by a slow component, width ∼100 μs, delayed relative to the laser pulse. The fast component exhibits atomic transitions of I(I) and I(II). The slow component reproduces the B̃←X̃ photoacoustic (PA) spectrum of molecular I2. The signal delay of the slow component accords with the velocity of acoustic waves in iodine vapor. The rf electrode region is the ‘‘sensitive’’ region where the acoustic wavegenerates the slow LOG signal. Two mechanisms of signal generation and propagation are involved. The fast signal originates in a two‐step laser photoionization of plasma‐excited atoms, the first‐step being resonant, and/or in changes of the atomic collisional ionization rates. These processes occur on time scales shorter than the laser pulse and generate an ‘‘instantaneous’’ LOG signal by creating additional electron–ion pairs. The delay of the molecular LOG signal, which is mediated by the PA effect, indicates that local heating produces a negligible perturbation of the discharge impedance. This is contrary to common belief. The polarity of the slow LOG signal depends on the direction of the PA wave, suggesting that the signal is generated by an actual physical movement of charged species by the pressure wave. Thermal effects are involved, but only as precursors to the PA wave.
Time‐dependent Hartree wave packet dynamical techniques for computation of electronically excited state optical spectra of many‐body quantum systems90(1989); http://dx.doi.org/10.1063/1.455812View Description Hide Description
An approximate solution technique for computing spectra of many‐body molecular systems is proposed. We focus for concreteness on 0 K electronic absorption and emission spectra. From a time‐domain perspective, it is necessary to propagate a well‐defined initial Schrödinger wave packet on a specified potential energy surface in order to extract such spectra. In order to perform this task for systems with many degrees of freedom, we investigate the utility of a time‐dependent Hartree factorization, in which the wave packet for the complete system is variationally factorized into a product of wave packets of smaller dimensionality. This method is shown to be both flexible and reliable for prototypical model systems associated with the physical problem of impurity spectra in host crystals. Successful application is made to a recently measured emission spectrum of I2 embedded in an argon matrix.
The A 2Σ+ state of rare gas–NO van der Waals molecules probed by 1+1 multiphoton ionization spectroscopy90(1989); http://dx.doi.org/10.1063/1.455813View Description Hide Description
The previously unobserved bound–bound spectra of ArNO, KrNO, and XeNO have been observed slightly blue shifted from the (1,0) and (0,0) bands of the A 2Σ+←X 2Π1 / 2 transition of uncomplexed nitric oxide. Although the structured but incomplete spectra cannot be assigned with certainty, limits to the ground and excited state bond dissociation energies,D ‘ 0 and D 0, respectively, can be estimated. For ArNO these limits are D ‘ 0 ≥89 and 54 cm− 1≤D 0 ≤101 cm− 1. The observed fragmentation of the KrNO and XeNO molecules, coupled with earlier results for MPI via the C 2Π state, suggests that superexcited, autoionizing states of the van der Waals molecules dissociate to yield excited rare gas atoms whenever energetically possible.
Angular correlation between photofragment velocity and angular momentum measured by resonance enhanced multiphoton ionization detection90(1989); http://dx.doi.org/10.1063/1.455814View Description Hide Description
REMPI detection of photofragments is discussed as a probe technique to measure the angular distribution of the velocity as well as the angular correlation between velocity and angular momentum existing in fragments formed during a polarized photolysis. The symmetry properties of the REMPI probe process induced by polarized light are expressed in terms of sensitivity to the moments of the rotational angular momentum distribution and the angular state of the photofragment is described in the formalism of bipolar moments. Original explicit expressions of the ion velocity profiles are given with probe geometry characteristics as parameters for two kinds of fragment detection (time‐of‐flight mass spectrometry and angular measurements of the ion yield) and the consequences of the v–Jcorrelation on the observed profiles are discussed. Some experimental geometries are proposed in order to either determine the angular correlation or to avoid its effects.
90(1989); http://dx.doi.org/10.1063/1.456663View Description Hide Description
Molecular beam electric resonance spectroscopy has been used to study HCCH–CO2 and DCCD–CO2, giving the following results for HCCH–CO2: A=8875 MHz, B=2861.45 MHz, C=2156.25 MHz, D J =12.4 kHz, D J K =36.7 kHz, and d 1=−3.25 kHz. The permanent dipole moment is 0.1611 D. The equilibrium geometry has the monomers parallel to one another in a configuration having C 2v symmetry. Deuterium e Q q data provide information on rms vibrational amplitudes and also gives an estimate of 40 cm− 1 for the out of plane bending mode. The monomer–monomer stretching vibration is estimated to be 75 cm− 1 from the D J measurement.
Raman band shapes of simple molecules dissolved in liquid perfluorocompounds: Solute–solvent interaction study90(1989); http://dx.doi.org/10.1063/1.455763View Description Hide Description
The vibrational Raman band shapes of N2, O2, and CO2 dissolved in perfluorotripropyl amine (FTPA), perfluorodecalin (FDC), and perfluorobenzene (FB) have been measured under elevated pressure (10–55 atm.). The full width at half‐height (FWHH) of N2 and O2 vibrational spectra in the perfluoro‐solvents increases with the order FTPA<FDC<FB. The vibrational bandwidths of the CO2 Fermi diad show explicit concentration dependence on these perfluoro‐solvents. The FWHHs decrease with decreasing concentration. The anisotropicI V H band shape of CO2 ν1 mode in solution state, which tends to broadening when CO2 concentration decreases, is reported. The solvent and concentration dependences of the vibrational and rotational band shapes can be rationalized on the basis of cavity structure of the liquid perfluorocompounds and of no specific interaction between the solute and the solvent.
Measurement and prediction of Raman Q‐branch line self‐broadening coefficients for CO from 400 to 1500 K90(1989); http://dx.doi.org/10.1063/1.455764View Description Hide Description
The J and temperature dependence of the self‐broadening coefficients for the Raman Q‐branch lines of pure CO have been experimentally determined for Q(J) transitions with J=0–38 and for temperatures in the range 400–1500 K. It is shown that a fitting law, based on a modified exponential energy‐gap model for the rates of state‐to‐state rotationally inelastic collisions, can account for the observed J dependence. The two parameters that determine the J dependence are found to be essentially independent of temperature. A temperature scaling function, recently proposed for N2, is added to the basic rate law, and accurate predictions of both the J and the T dependence of these coefficients and those previously reported at 298 K are obtained. This rate law model, used in conjunction with a relaxation matrix description of the Q‐branch spectrum, is shown to give good agreement with the observed, partially collapsed spectrum at 2.8 atm and 295 K.
Rotational spectra and structures of small clusters containing the HCN dimer: (HCN)2–Y with Y=HF, HCl, HCF3, and CO290(1989); http://dx.doi.org/10.1063/1.456661View Description Hide Description
Microwave rotational spectra have been observed for a number of isotopic species of the (HCN)2–HF, –HCl, –HCF3, and –CO2 trimers. The observations were made with the pulsed nozzle, Fourier transform, Flygare/Balle Mark II spectrometer. The trimers have structures which are composites of the linear (HCN)2 dimer and the HCN–Y dimers, the latter linear for Y=HF and HCl, a symmetric top for Y=HCF3, and T‐shaped with C 2 v symmetry for Y=CO2. The rotational constants for the most abundant species of each trimer are as follows: For Y=HF and HCl, B 0 is 699.204 and 467.408 MHz, respectively, and D J is 162 and 87 Hz; for Y=HCF3, B 0 is 305.742 MHz and D J and D J K are 51 and 471 Hz; for Y=CO2, treated as a symmetric top, (B 0+C 0)/2 is 452.426 MHz and D J is 1.057 kHz. Hyperfine interaction constants were determined for several species. The B 0’s for each trimer were analyzed by a combination of isotopic substitution and fitting procedures to determine the distances r 1 and r 2 between the centers of mass (c.m.) of adjacent monomers. The B 0’s are relatively insensitive to the position of the central HCN but give r 1+r 2 accurately. With this limitation, r 1 and r 2 in the trimers are compared with the corresponding distances in the dimers, which are longer. For Y=HF, HCl, HCF3, and CO2, respectively, the shrinkages found in r 1 are 0.069, 0.054, 0.030, and 0.004 Å and in r 2, 0.043, 0.062, 0.042, and 0.052 Å. The shrinkage in r 1 and several other properties of the trimers exhibit some correlation with the pseudodiatomic stretching force constant in the HCN–Y dimer.
Structure dependent competition between multiphoton absorption and intramolecular energy redistribution in organomolybdenum compounds: State selective production of molybdenum atoms90(1989); http://dx.doi.org/10.1063/1.455765View Description Hide Description
We have measured one color multiphoton dissociation/ionization spectra (MPD/MPI) across the one photon region 23 809 to 28 735 cm− 1 for a structurally homologous series of organomolybdenum molecules including: (η6‐cycloheptatriene)Mo(CO)3, Mo(CO)6, and a variety of (η6‐arene)Mo(CO)3‐type molecules. The degree of observability of the 7 D J , 7 P 0 J , and 7 S 3Rydberg series is observed to be a function of: (1) the number of low frequency vibrations associated with the molecular precursor, and (2) the degree of localization of the cyclic ligand to metal bond. These Rydberg series are taken as a simple but sensitive probe of the production of ground state metal atoms. The efficiency for MPD production of 7 S 3molybdenum atoms relative to (η6‐benzene)Mo(CO)3 increases in the order 1:1.9:11 with addition of a methyl substituent and extending it to an ethyl group. These observations are qualitatively and quantitatively in good agreement with previously reported relative efficiencies of MPD/MPI of organochromium molecules to produce ground state chromium atoms.